On the other hand, devices are already known for screwing screw caps onto containers, in which devices, the closing head can be driven directly by its own motor with adjustable torque (DE 40 11 398 C2). Here, the speed of rotation of the closing head is monitored by an angle of rotation sensor associated with it. This angle of rotation sensor controls, on the one hand, the drive motor for the closing head and, on the other hand, it generates, in connection with an evaluation device, an error signal when the closing head turns or does not turn at certain places. This leads to the removal of the corresponding incorrectly closed container.
The invention is based on the problem of providing simple means for monitoring the closing head rotation in a device of the type mentioned in the introduction.
In a device according to the invention, the permanent magnets, which are present anyway, of the slide or hysteresis coupling are used to monitor the turning of the closing head. Based on this double function of the magnet, according to the invention, the expenditure for the construction is extremely small. The magnets, in general, are arranged with good protection in a housing, so that a microbiologically advantageous construction of the closing device is not negatively affected.
In the simplest case, the detector has a coil, optionally with a soft iron core, which is located opposite the closing head. During a relative rotation between the coil and the magnet, a voltage is induced in the coil, which can be evaluated in a simple manner in a connected evaluation device.
If several devices are arranged on a common rotor, then the detector can be arranged, without problem, in a stationary position in a certain area of the circular path of the closing heads. Depending on whether or not the closing heads undergo, in addition to the circular motion, a rotation of their own, a different signal is then produced in the detector, which again can be evaluated in a simple manner.
The detector may, but does not have to, cover the entire circular path of the closing heads. According to a preferred variant of the invention, it merely monitors the area in which, during the normal course of the closing process, after the tightening of the screw cap with the set torque, the magnetic coupling of the closing head no longer undergoes a rotation of its own. However, if in that area, a rotation of the magnet itself is registered, then this indicates, among other facts, one or more of the following errors or defects:                a) The container continues to turn because of a defective or worn clamp;        b) The closing head continues to turn because of a worn toothing of the closing cone;        c) The screw cap continues to turn because of a defective threading on the closure or container.        
All these cases, which lead to an incorrect fit of the screw cap, can be acquired by the stationary detector. By means of an evaluation device which is connected to the latter detector, it is then possible to trigger, as desired, for example, an alarm signal, to automatically remove the container with the defective closure from the circulation and/or to identify the closing head which has caused the incorrect fit.